There are two main tire nomenclature methods...metric, and SAE.
They are mixed together on the same tire, just to make you confused though.
For example, the classic SAE Light Truck (LT) sizing typically uses the overall diameter as the first number, then, the section width, then the rim size it fits on.
So a 31x10.5/15 tire is NOMINALLY 31" in diameter, has a 10.5" section width, and fits onto a 15" rim.
Its NOMINAL for the tire, as in its ABOUT that, but not exactly that...off by up to an inch or more depending on the sizes involved.
So a 31x10.5/15 tire MIGHT be 30"-32" in diameter, and 10-11" in section width (The 15" for the rim is exact though)
Its a ball park sort of sizing scheme....and so is the metric system.
The metric system never TELLS you the diameter....it only tells you the section width, the aspect ratio, and the rim size.
The section width is still sidewall to sidewall as it is for the LT sized tires (NOT the tread width), but in millimeters (mm) instead of inches.
There are 25.4 mm in an inch
This means that a 10.5" sidewall is 10.5 x 25.4 mm in metric measurement, or 266.7 mm
266.7 mm is = 265 mm NOMINAL....so, the above tire would be a 265 mm section width to be like a 10.5" section width LT tire, etc.
The ASPECT RATIO of a tire is really a percentage of how tall the SIDEWALL is relative to the SECTION WIDTH.
The aspect ratio is expressed as a 2 digit number, such as 70 or 75, etc.
If the sidewall is 75% as tall as the section width is wide, then its aspect ratio is 75.
These are also nominally expressed in multiples of 10 or 5, such as 50, 55. 60, 65, 70, 75, 80, etc....rounded to match the above.
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So, for a metric size of 265/75/16:
265 = section width, in mm
75 = aspect ratio
16 = rim size
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To see the DIAMETER of the 265/75/16 tire, you need to use the above as follows:
Find the sidewall height, add the sidewall HEIGHTS to the rim diameter, and that's the TIRE diameter.
TO find the sidewall height, take 75% of the 265 mm section width.
265 mm x 0.75 = 198.75 mm
Convert to inches:
198.75/25.4 = 7.8" per sidewall.
Add the 2 sidewall heights (One over the rim, one from the rim to the ground...) to your 16" rim, to get the overall tire diameter:
7.8" x 2 = 15.6" of total sidewall heights
15.6" of sidewall + 16" of rim = 31.6" of total tire diameter.
So, a 265/75/16 tire is essentially a 32 x 10.5/16 LT sized tire.
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So, generally, the LT tires are sized:
30x9.5/15
31x10.5/15
32x11.5/15
33x12.5/15
and so forth.
These would be considered as "Normal" 30's, 31's, 32's, 3's, etc.
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As a general rule, metrics use the following section widths:
245
255
265
275
285
295
305
315
With most of the aspect ratios for our trucks being 65 - 80.
The lower the aspect ratio (65 is lower than 75, and so forth), the shorter the sidewall, for a given section width.
The larger the section width, the wider the tire.
So - you can make a tire taller by ONLY changing its section width, or, by ONLY changing its aspect ratio....or with both.
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The metrics tend to be more accurate in size description, but are still nominal...so one tire (Same brand and model) might be a 305 section width, and another might be a 315 section width, but, the ACTUAL section width might be the same.
One 265/75/16 might be taller or shorter than another, or wider or skinnier, and so forth.
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LT sized tires are more conservatively load rated....for example, if your truck CAME with LT's, and you looked up the LOAD RANGE for the LT it CAME WITH, but, you were replacing it with a metric sized tire, you need to raise the load capacity by 10-15% over the LT's spec, because the LT's spec INCLUDED some fudge factors unique to trucks, but the metric tire did NOT.
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Hope that helps!